Force development during striated muscle contraction is initiated by the binding of Ca2+ to the specific sites in troponin C (TnC), triggering a series of functional structural changes within the thin filament, including opening of the N-domain of TnC, conformational change of the inhibitory region of troponin I (Tnl), and switching interaction between Tnl and actin to Tnl and TnC, which ultimately lead to a cyclic interaction between actin and myosin to form strong force-generating cross-bridges. Full muscle activation requires both Ca2+ binding and feed back modulation of cross-bridge cycling. In cardiac muscle it is also modulated by protein phosphorylation which plays important roles in heart failing/hypertrophic process. To fully understand muscle regulatory mechanism requires structural, thermodynamic and kinetic information on each of these structural transitions during force development. My long-term research goal is to elucidate the kinetics of movements of the thin finlament betweem extremes of contraction/relaxation, and understand how they are modified by cross-bridge cycling and phosphorylation. To achieve the goal, this proposal addresses the following three issues: (1) What is the kinetics of each individual activation/deactivation process of the thin filament? (2) How does the cross-bridge cycling affect these kinetic processes? And (3) what is the role of phsophorylation in modulating these transitions? Newly designed conformational markers based on Forster resonance energy transfer to monitor these structural transitions will be used for stopped-flow kinetic and Ca2+ titration measurements at different activation conditions to acquire the desired information. These markers will be incorporated into reconstituted thin filament, myofibrils and skinned fibers along with/without phosphotylated proteins to specify the time-dependent changes of specific domain movements of the thin filament in response to Ca2+. Results of this study will enhance our understanding of molecular mechanisms of thin filament activation in response of Ca2+ and the role of protein phosphorylation in heart failure.